1. Field of the Invention
The present invention relates to improvements in belt-type continuously variable transmissions comprising a metal belt including pieces for automobile and realizes a structure which stabilizes the friction coefficient of belt with pulley, inhibits the effect of collision of elements constituting the belt with each other and resonance of the belt with the elements and the pulley during transmission, gives a high transmission efficiency and prolonged life even when a low viscosity CVT fluid or ATF oil is used for high fuel efficiency, and prevents early exfoliation of rolling bearing for bearing pulley and rolling bearing disposed in units during their operation.
2. Description of the Related Art
In recent years, there has been a growing demand for higher automobile fuel efficiency from the standpoint of environmental protection. The recent trend is for more belt-type continuously variable transmissions (B-CVT) having a higher efficiency to be used rather than related art multistage automatic transmission (AT). Various belt-type continuously variable transmissions have been developed. For example, the metal belt-type continuously variable transmission 1 schematically shown in FIG. 1 has an input side rotary shaft 10 and an output side rotary shaft 20 which are disposed in parallel to each other. The input side rotary shaft 10 is born by a pair of rolling bearings 11, 11 and rotationally driven by an engine 30 via a torque converter 31 and a start clutch such as solenoid clutch 32. On the other hand, the output side rotary shaft 20 is rotatably born by a pair of rolling bearings 21, 21 inside a transmission case (not shown). The rotation of the output side rotary shaft 20 is transmitted to a pair of right and left drive wheels 25, 25 via a reduction gear train 22 and a differential gear 23.
Disposed in the middle portion of the input side rotary shaft 10 is a driving pulley 40 so that the driving pulley 40 and the input side rotary shaft 10 rotate in synchronism with each other. The clearance between a pair of driving pulley plates 41, 41 constituting the driving pulley 40 can be freely adjusted by a driving dislocation unit 42. In other words, the groove width of the driving pulley 40 can be freely raised or reduced by the driving dislocation unit 42. On the other hand, disposed in the middle portion of the output side rotary shaft 20 is a driven pulley 50 so that the driven pulley 50 and the output side rotary shaft 20 rotate in synchronism with each other. The clearance between a pair of driven pulley plates 51, 51 constituting the driven pulley 50 can be freely adjusted by a driven dislocation unit 52. In other words, the groove width of the driven pulley 50 can be freely expanded or shrunk by the driven dislocation unit 52. An endless metal belt with pieces 60 extends over the driven pulley 40 and the driving pulley 50. The metal belt with pieces 60 comprises an endless combination of a number of piece shaped metallic elements.
In operation, the belt-type continuously variable transmission 1 comprising a metal belt with pieces 60 having the aforementioned constitution allows the power transmitted from the engine 30 to the input side rotary shaft 10 via the start clutch to be transmitted to the driven pulley 50 via the metal belt with pieces 60. As examples of the metal belt with pieces 60 there has heretofore been known one which transmits power in the pushing direction and one which transmits power in the pulling direction. In any case, the power transmitted to the driven pulley 50 is then transmitted from the output side rotary shaft 20 to the drive wheels 25, 25 via the reduction gear train 22 and the differential gear 23. In order to change the reduction ratio from the input side rotary shaft 10 to the output side rotary shaft 20, the groove width of the driving pulley 40 and the driven pulley 50 are raised or reduced in relation to each other.
For example, in order to raise the reduction ratio from the input side rotary shaft 10 to the output side rotary shaft 20, the groove width of the driving pulley 40 is raised while the groove width of the driven pulley 50 is reduced. As a result, the diameter of the circle formed by the metal belt with pieces 60 on the pulleys 40, 50 is small on the driving pulley part and large on the driven pulley part, giving a reduction ratio from the input side rotary shaft 10 to the output side rotary shaft 20. On the contrary, in order to raise the multiplication ratio (reduce the reduction ratio) from the input side rotary shaft 10 to the output side rotary shaft 20, the groove width of the driving pulley 40 is reduced while the groove width of the driven pulley 50 is raised.
For transmission between the element formed by the piece metal constituting the metal belt with pieces 60 and the driving and driven pulleys 40, 50, the belt frequency f (Hz) can be represented by the equation f (Hz)=Zb×Nb/60 in which Zb represents the number of belt elements and Nb represents the rotary speed of belt. For example, in the case where the number of belt elements (metallic tops) is from 250 to 400, when the rotary speed of the engine is changed from 600 min−1 to 7,000 min−1, the primary shaft shows a phenomenon that the primary component of the frequency of vibration caused by the running of the belt is from 1,000 to 3,000 Hz during deceleration but in a range as high as 10,000 to 35,000 Hz during acceleration. On the other hand, MT and AT show a phenomenon that the primary frequency of vibration caused by the engagement of gears is lower on both low and high gear ratio sides than belt-type continuously variable transmissions because they normally have 50 or less gears.
Another characteristic of the belt-type continuously variable transmission 1 comprising a metal belt with pieces 60 is thought that the friction coefficient of the metal belt with pieces 60 with the driving and driven pulleys 40, 50 changes by a range of from 0.1 to 0.15 unlike MT and AT. Since actual belt running involves repetition of multiplication and reduction, resonance attributed to the metal belt with pieces 60 can occur. This resonance frequency is determined by the length of the metal belt with pieces 60 and the tension of the belt. However, since the actual operation gives a vibration having a wide range of frequency, a high frequency acts on the interior of the unit, particularly the rolling bearings 11, 21 for bearing the driving and driven pulleys 40, 50, respectively, when the resonance frequency of the belt is often passed.
Accordingly, since the friction coefficient of the metal belt with pieces 60 with the driving and driven pulleys 40, 50 is normally raised for stabilization, a CVT fluid (also for ATF) having a friction coefficient of not smaller than 0.07 is supplied at a rate of not smaller than 300 cc/min. However, since the rolling bearings 11, 21 for bearing the driving and driven pulleys 40, 50, respectively, are disposed on the side of the pulleys 40, 50, respectively, they can be difficultly lubricated thoroughly and are subject to violent vibration due to the resonance of the belt or passage of the elements which are metallic tops, occasionally causing local deterioration of oil film formed on these rolling bearings 11, 21. Accordingly, it is necessary to consider the bearing design such as increase of the supplied amount of lubricant, increase of the bearing size and increase of the ball diameter for the purpose of increasing basic dynamic nominal load.
The recent trend is that a CVT fluid having a raised fluidity and a reduced viscosity is used to provide the belt-type continuously variable transmission 1 with a desired efficiency and suppress the noise occurring in operation while inhibiting the abrasion on the driving and driven pulleys 40, 50 and the metal belt with pieces 60. Therefore, it is thought that standard rolling bearings suffer early exfoliation due to insufficient oil formation attributed to slippage between bearing ring and rolling elements combined with violent axial vibration accompanying the belt resonance. However, a rolling bearing having an inner ring, an outer ring and rolling elements made of ordinary bearing steel is operated at a bearing temperature of higher than 100° C. lubricated with a low viscosity CVT fluid (dynamic viscosity of basic oil: 40 mm2/sec or less at 40° C. or 10 mm2/sec or less at 100° C.). Thus, the amount of lubricant to be supplied into the interior of the bearing falls below the expected value (mallubrication). FIG. 2 is a graph illustrating the fatigue pattern of a belt-type continuously variable transmission. FIG. 3 is a graph illustrating the fatigue of an ordinary T/M bearing. Due to this mallubrication, belt-type continuously variable transmissions exhibit a surface fatigue of higher than 2.0 in a short period of time. This is attributed to the fact that the effect of slippage such as differential movement, revolution and spin causes break of oil film and thus makes the raceway surface fresh, accelerating fatigue. This early fatigue has made the related art belt-type continuously variable transmissions disadvantageous in respect to occurrence of early exfoliation.
The analysis of the aforementioned fatigue is conducted on the basis of fatigue parameter F (=ΔB+K×ΔRA (in which ΔB represents the reduction of half-width, K represents a constant depending on the material used, and ΔRA represents the reduction of retained austenite) as disclosed in JP-B-63-34423. In some detail, X-ray diffraction half-width of martensite phase and retained austenite content (vol-%) before and after fatigue of the rolling portion of the metallic material are measured. From these measurements are then determined the difference ΔRA between retained austenite content (vol-%) before fatigue and retained austenite content after fatigue and the difference ΔB between X-ray diffraction half-width of martensite phase before fatigue and X-ray diffraction half-width of martensite phase after fatigue. These values are then substituted in the aforementioned equation, respectively, to determine the fatigue parameter. The fatigue parameter thus determined is then evaluated on the previously prepared criterion depending on the various sites of the rolling portion to effect analysis.
Further, the local break of oil film on the contact surface of the raceway with the rolling elements results in the exposure of highly active fresh surface where the additives, etc. in the lubricant exert catalytic action that causes the decomposition of the lubricant or water content in the lubricant to hydrogen which then penetrates and diffuses in the steel and is accumulated in the stress sites (sites in the vicinity of the maximum shear stress in the surface layer) to drastically deteriorate the resistance of the steel. FIG. 4 indicates the measurements of the amount of hydrogen in the balls before and after rotation of deep-groove ball bearing 6206 made of steel with a commercially available CVT fluid at a high temperature (120° C.) for a predetermined period of time. It can be recognized that hydrogen penetrates in the steel material during rotation.
In order to prevent the penetration of hydrogen, the formation of an Ni deposit on the rolling surface has been proposed (see Society of Automotive Engineers of Japan, Inc.'s Symposium Preprint No. 30-02, pp. 5–8, 2002). However, since an Ni deposit is soft, it is thought that the Ni deposit falls off the steel due to abrasion and thus cannot exert its effect sufficiently under conditions that a lubricant having a low viscosity is used to cause break of oil film due to violent vibration, load variation, slippage, etc.
As the rolling bearings 11, 21 to be incorporated in the belt-type continuously variable transmission 1 there have heretofore been used ones formed by inner ring, outer ring and balls obtained by subjecting SUJ2 to hardening and tempering to HTC of from 58 to 64. However, these rolling bearings are subject to early exfoliation as mentioned above. The present applicant early proposed a rolling bearing for belt-type continuously variable transmission having an exfoliation resistance improved by forming inner ring, outer ring and rolling elements by a steel material having finely divided molybdenum-based carbides or vanadium-based carbides separated out in dispersion therein and hence less occurrence of eutectic carbides (see JP-A-2000-328203).
However, the demand for further enhancement of the belt-type continuously variable transmission 1 is inevitable. There has been a growing demand for excellent exfoliation resistance of various rolling bearings to be incorporated in the belt-type continuously variable transmission 1. It has been further required that exfoliation due to penetration of hydrogen be coped with sufficiently.